The Study
A 40-unit apartment building in Auckland, certified to a 7-Homestar rating, was studied by researchers from the University of Auckland and Massey University in a project titled “What is Drier? Understanding Humidity in Green-Certified Dwellings.” The study population was residents aged 65 and older — a group considered more vulnerable to indoor air quality and humidity issues. The finding that challenged conventional assumptions: relative humidity in the building often exceeded the commonly recommended 40 to 60 percent range, but most residents reported feeling comfortable, and absolute humidity remained within healthy parameters.
The Relative vs Absolute Humidity Distinction
This distinction is at the heart of the research’s significance. Relative humidity measures the amount of moisture in the air as a percentage of the maximum moisture the air could hold at that temperature. Absolute humidity measures the actual mass of water vapour per unit volume of air, regardless of temperature. The two measures diverge because warm air can hold more moisture than cold air at the same absolute humidity level.
In Auckland’s coastal climate — warm and humid — outdoor air brought inside through ventilation may carry high relative humidity readings while the absolute moisture content remains within the range associated with good health outcomes. Dr Mike Rehm, the study’s lead researcher, stated it plainly: “Humidity and dampness are often used interchangeably, but they are not the same.”
The Implication for Green Building Standards
The research suggests that applying relative humidity thresholds developed for continental climates to coastal New Zealand conditions may not produce the outcomes those thresholds were designed to achieve. In Auckland’s coastal environment, natural ventilation may not reduce relative humidity effectively — and installing mechanical systems to drive relative humidity below the threshold may be unnecessary if absolute humidity levels are acceptable.
The study’s recommendation is that building performance standards should incorporate absolute humidity measurement alongside relative humidity, and that guidelines be adapted to New Zealand’s diverse regional climates rather than applying uniform standards derived from international references.
Implications for Builders and Designers
For builders specifying ventilation systems for residential projects in coastal New Zealand — particularly in Auckland and Northland — the research provides a basis for more nuanced specification decisions. Mechanical ventilation systems that achieve code compliance on relative humidity targets may not be necessary or appropriate in all coastal contexts. Engaging with the research findings, and with NZGBC’s guidance as it evolves to reflect them, will help produce better-performing homes rather than technically compliant ones that do not serve their occupants well.
